This invention relates to switched reluctance machines and more particularly to control systems and methods for such machines not having rotor position sensors.
Switched reluctance motors are doubly salient motors, that is they have teeth on both the stator and the rotor. The stators of such motors have windings to form phases of the motor, but the rotor has no windings. The phases of the stator are energized sequentially in a cyclical fashion so that a magnetic force of attraction occurs between the energized stator pole and the rotating rotor. This current must be switched on and off at the proper times to provide the proper attraction between the rotor poles and the energized stator pole without producing a negative or braking attraction once the rotor reaches its aligned position with the stator. Because of the necessity of switching these phase energization currents on or off in synchronism with the rotary position of the rotor, it has been common in the past to utilize a shaft position sensor to determine the rotary position of the rotor. For example, optical shaft encoders have been used as such shaft position sensors. However, because the optical shaft encoders or other shaft position sensors require space in the motor and because of the necessity of running signal wires from the shaft position sensor to the control circuitry, shaft position sensors are not without their disadvantages in switched reluctance motors. This disadvantage has been overcome in the past by driving the switch reluctance motor in a way similar to a stepping motor. The motor is supplied with constant voltage from a direct current (DC) source with sequences of half-cycle rectangular voltage pulses applied appropriate to the phase number. The torque produced with a such a system is higher than needed. That is, there is a large torque margin, which is not economical for large units.